Young organisms are usually acknowledged to respond more profoundly than adults to toxic agents. Much of the evidence for this assumption is based on comparisons between the earliest periods of development and the period of young adulthood. Later in the life cycle, however, diminished capacities, reminiscent of immaturity, appear once again and are expressed in a comprehensive spectrum of functional deficits. These consequences of aging are almost surely accompanied by enhanced vulnerability to toxic challenge, and pose serious policy and scientific questions to the environmental health sciences. The relationships between the consequences of aging and how they modify the consequences of toxic exposure will be explored in rats treated chronically with low levels of inorganic lead. The experiments will rely on both cross-sectional and longitudinal designs to span a range of ages from 1 to 32 months. Behavioral measures will supply the primary indexes of function, and will be based on an extension of schedule-controlled performance, the Fixed-Interval schedule, shown previously to be sensitive to low levels of lead. These measures are designed to provide assays of motor function and adaptability as well. The behavioral measures will be accompanied by a variety of biological measures. These include: blood and tissue lead concentrations, assays of kidney function, and histopathology. Reversibility will be determined in two ways, by cessation of exposure and by treatment with a chelating agent.